1. Field of the Invention
The invention relates to a process for preparing modified silver catalysts which can be used in oxidation processes, especially in a process for the production of ethylene oxide by direct oxidation of ethylene with molecular oxygen.
2. The Prior Art
Catalysts comprising from 1 to 35% by weight of silver on a porous refractory support are known to be useful for the production of ethylene oxide by the controlled incomplete oxidation of ethylene with molecular oxygen. A great variety of modifications have been proposed to improve the activity and selectivity of silver catalysts. The modifications have involved, for example, the supports employed, the method of production, the physical form of the silver on the support, and the addition of certain additives to the catalyst.
It has been observed that the selectivity of silver catalysts may change during the initial period of their use in the manufacture of ethylene oxide. The selectivity of some catalysts increases during this period of operation, and then remains constant for a long period of time, whereas the selectivity of other catalysts gradually decreases, and then remains constant or fairly constant. The applicants have found that this change in initial activity is probably due to stabilization of the silver catalyst taking place during the initial period of their use.
It is known from British patent specification No. 1,413,251 that the addition of from 0.00035 to 0.0030 gram equivalent weights per kilogram (based on the entire catalyst) of ions of one or more of the alkali metals potassium, rubidium or cesium, coincidentally with the deposit of silver on the catalyst support results in an improved selectivity of the catalyst thus obtained. According to this British patent specification the alkali metal and the silver are deposited coincidentally on the catalyst support since in this way catalysts having optimum selectivity (75-81%) are obtained. From Example VII of British patent specification No. 1,413,251 it appears that when potassium is deposited on the support after the deposition of the silver, the selectivity improvement is smaller than that obtained by simultaneous deposition of silver and alkali metal. The silver catalyst used in this example was prepared by impregnating a support with a solution of a silver oxalate complex followed by heating in order to reduce the silver salt to silver metal. This silver catalyst showed a selectivity of 69%. After the treatment with the potassium-containing solution the selectivity was increased to 73-74%, which is clearly less than the optimum mentioned above.
Hoechst, Inc., filed a German patent application on May 2, 1975, now issued as German Pat. No. 2,519,599, July 1, 1976 which discloses a process in which so-called "tired" silver catalysts which have been used for the manufacture of ethylene oxide for a long period of time are soaked with an impregnating solution consisting of (1) 0.2 to 5% w of water, (2) 0.05 to 0.4% w of cesium or rubidium nitrate, and (3) an aliphatic alcohol having 1 to 3 carbon atoms, and the alcohol is subsequently removed at a temperature between 70.degree. and 120.degree. C., preferably between 90.degree. and 110.degree. C., optionally with blowing through of nitrogen. There is no indication that other solvents or anions other than nitrate are operative.